1. Field of the Invention
The present invention relates to an a.c. power supply apparatus such as an uninterruptible power supply (UPS) or a fuel cell power generation apparatus.
2. Description of the Prior Art
FIG. 1 shows the arrangement of a typical conventional UPS. In the figure, a charger 3 converts a.c. power of frequency f.sub.2, received through a transformer T.sub.1 from a commercial power source 5, into d.c. power, and supplies the d.c. power to a voltage-type inverter 1 while charging a battery 2.
The inverter 1 converts the d.c. power into a.c. power of frequency f.sub.3 including a less amount of low-order harmonics. The a.c. power is made sinusoidal by being fed through a filter constructed of an inductor L.sub.s and capacitor C.sub.p, and, by being fed through a transformer T.sub.2 so as to meet a load voltage, it is supplied to a load 4.
Generally, the load of a UPS, e.g., a computer, is isolated from the power source line for protection against noise and is provided with an exclusive ground, in most cases, and therefore the transformer T.sub.1 is required for the purpose of power line isolation as well as voltage matching.
Although, in some cases, the input power transformer T.sub.1 is omitted, the charger 3 has its d.c. output voltage determined on the basis of economy of the inverter 1 and battery 2, and therefore the transformer T.sub.1 is used to provide a proper voltage for the inverter and also to isolate the load line from the power source line, in most cases.
Accordingly, a conventionally designed UPS necessitates two heavy bulky transformers, and thus preventing the reduction in the size and weight of the UPS.
An advanced apparatus which has been developed with the intention of overcoming the above problem is the high-frequency intermediate link method power supply as shown in FIG. 2. The UPS apparatus shown in FIG. 2, which functions similar to that of FIG. 1, is based on the high-frequency intermediate link DC/AC inverter disclosed in an article entitled "Classification of Inverters and Their Characteristics", Electric Review, FIG. 14, pp. 987-992, November 1981.
In the figure, inverter 1 is a voltage-type inverter producing a single-phase rectangular waveform of f.sub.1 =10 kHz for example, and it supplies the output to a cycloconverter 6 through a transformer T.sub.2 for isolation. The cycloconverter 6 converts the frequency of a.c. power from f.sub.1 to f.sub.3 =60 Hz for example, and the power is fed through a filter constructed of an inductor L.sub.s and capacitor C.sub.p, so that it becomes sinusoidal, and supplied to a load 4. This apparatus has its transformer T.sub.2 designed to operate at 10 kHz, and therefore it can, be compact and light weight. However, the apparatus still necessitates a transformer T.sub.1 of the commercial power frequency f.sub.2 for a charger 3, as in the case of FIG. 1.
FIG. 3 shows a more advanced apparatus, in which the same high-frequency intermediate link method is further applied to the charger 3 in consideration that the DC/AC inverter of FIG. 2 is reversible. Although this apparatus can have compact transformers, power is transmitted through two cycloconverters 6 and 8 and two inverters 1 and 7 between the commercial power input and power output, resulting in a degraded efficiency and increased cost of an additional converter. Accordingly, the apparatus of FIG. 3 is less practical from the viewpoint of economy and efficiency.
A more innovative apparatus intended to overcome the above deficiencies is offered in an article entitled "Small UPS Using Phase Control", INTELEC '87 Conference Proceedings, Session 12, FIG. 16(b) in pp. 516-520. The apparatus, which does not necessitate a charger, is shown in FIG. 4 in the same depictive manner as of FIGS. 1 through 3.
In this apparatus, when the commercial power source 5 is normal, a.c. power of frequency f.sub.2 from the commercial power source 5 is directly supplied through a switch SW to the load 4 and at the same time fed to a cycloconverter 6 through a filter formed of an inductor L.sub.s and capacitor C.sub.p. The converter 6 converts the power to have a frequency f.sub.1, which is fed through a transformer T.sub.2 to an inverter 1, which then produces d.c. power to charge the battery 2.
When the commercial power source 5 is out, the switch SW is opened so that power of the battery 2 is fed through the inverter 1, transformer T.sub.2, cycloconverter 6 and filter and supplied to the load 4.
This apparatus is highly practical because of its need of only two converting devices, however, power with the same voltage and frequency as the commercial power source is supplied to the load 4, and therefore it is not suited to applications which require a precise voltage and constant frequency.